Conventional testing systems that measure device under test (DUT) voltages are used to determine if a DUT or a component thereof is operating properly. One such type of testing system is automated testing equipment (ATE) instrumentation. ATE instrumentation can include precision measurement units (PMUs) that are used to measure currents and voltages that are received or are output by DUTs. Some DUTs generate voltages that may exceed the voltage measurement range of conventional ATE PMU instruments. However, this causes problems as ATE PMU instruments cannot measure voltages from DUTs that exceed the measurement range of the PMU.
It should be appreciated that some conventional testing systems can only test voltage signals whose magnitudes fall below 15 volts for instance. However, increasingly, the testing of voltage signals of 30 volts or more can be required.
Test fixtures can be used to accommodate the performance of test operations in various types of test environments. Such test fixtures can be employed to provide an interface between test instrumentation, the DUT and the test operator. Some conventional test fixtures have been developed that include components that enable ATE PMUs to measure voltages that exceed their normal voltage range.
FIG. 1A shows a conventional circuit arrangement 100 that can be included as a part of a test fixture 109 that is used as an interface between a DUT 101 and a PMU 107 to enable the determination of DUT 101 output voltages that exceed the voltage measuring capacity of PMU 107. Shown in FIG. 1A are DUT 101, resistor1 103, resistor2 105 and PMU 107. Referring to FIG. 1A, resistor1 103 and resistor2 105 are connected in a typical voltage divider arrangement. This arrangement allows a voltage that is smaller than that which is located at the output of DUT 101 to be supplied as a reference to PMU 107. In this manner a voltage that lies within the voltage measuring range of PMU 107 can be supplied to PMU 107 from which the output voltage of DUT 101 can be computed.
The circuit shown in FIG. 1A has serious shortcomings. For example, because the system relies upon knowledge of the precise size of resistor1 103 and resistor2 105 as a means of accurately computing DUT 101 output voltages, resistors that have small tolerances should be used. This can present significant challenges because resistors in the mega ohm and giga ohm range can be necessary to generate sufficiently high voltages from the low power pins of typical DUTs. The actual resistance of such large resistors can vary widely, for instance a one mega ohm resistor with a tolerance of ten percent can have an actual resistance that is 100 kilo ohms more or less than the indicated value. Because of such lack of precision the accuracy of the voltage computations that are based on the listed resistance of such resistors can suffer greatly. Moreover, because the correct value of the resistor is relied upon, diagnostics and calibration to assure the resistor value is correct can be needed. In addition, these large resistors are expensive, hard to procure and are difficult to match (e.g., tolerances).
An additional shortcoming of the system shown in FIG. 1A is that the system draws current from DUT 101. Such draws of current can be detrimental to the operation of DUT 101 and can impact the accuracy of the measurement.
FIG. 1B shows another conventional circuit arrangement 100 that can be used as a part of a test fixture 113 that is used as an interface between a DUT 101 and a PMU 107 to enable the determination of DUT 101 output voltages that exceed the voltage measuring capacity of PMU 107. FIG. 1B shows in addition to the structures enumerated above with reference to FIG. 1A, buffer 111 (e.g., operational amplifier etc.) and test fixture 113. In the system shown in FIG. 1B, the draw of current from DUT 101 is obviated as current is supplied by buffer 111. However, a significant drawback of this system is that buffer 101 has to be powered with supply rails that have a voltage magnitude that is greater than the magnitude of the output voltage of DUT 107 in order to sense the output voltage of DUT 101. It should be appreciated that providing a rail that has such magnitude can present difficult design challenges especially in high voltage output ranges. Providing voltages, such as 25 volts or more, to the rails can be complex and costly.
In general, conventional approaches to measuring voltages that exceed the measuring range of testing systems can be expensive, place undesirable loads on the DUT or involve design challenges. Such approaches can be undesirable for many DUT testing applications.